This invention relates to a method for a color selection for color palettes for color expressions of color or images. More particularly, it relates to a method for a color selection in cases wherein the numbers of colors of color or palettes are changed stepwise.
As techniques for realization of digital color images for e.g. computer graphics, there are known a so-called full-graphics system in which the colors of the pixels are expressed using three color data of red(R), green(G) and blue(B), and a multicolor expression system in which a certain number of, for example, 16, colors, are previously selected from a large number of selectable colors, such as 4,096 colors, for so-called color palettes, and expression of color or images is achieved using only these selected colors. The multicolor expression system by the color or palettes has many advantages, such as a reduced number of bits of color or data for pixels as compared to the number of selectable colors, and reduced hardware cost and size.
Specifically, if four bits each are allocated to three color or data of R, G and B, the number of expressible colors is 2.sup.4 .times.2.sup.4 .times.2.sup.4 =4096 colors, while 12 bits are required as a color code for expressing a desired color. Thus, the desired 16 colors are selected from the 4096 colors to provide 16-color color palettes, and one of 16 color palette colors is expressed using four bits each for each pixel. Although the number of colors simultaneously selectable in one full picture of color images is 16, these 16 colors may be selected freely from the 4096 colors. The 4-bit numbers designating one of the 16 colors of the color palettes, or color numbers, and the 12-bit color codes, indicating to which of the 4096 colors the color numbers correspond, are grouped together in a so-called color lookup table. Although the color look-up table in the present example has a capacity of 4.times.16+12.times.16=32 bytes, it is apparent that the capacity of the image data in their entirety may be markedly reduced in consideration that 12 bits required for each pixel may now be reduced to four.
When the above described multicolor expression system by the color palettes is accepted as a standard for digital color image data recorded on a recording medium, a data format within a graphics device or in a data transmission format, the numbers of selectable colors or the numbers of color palette colors are usually set to predetermined values. If it is desired to extend the color palettes set by the prescribed standards in accordance with extension formats or upper order standards for increasing the number of the selectable colors or the number of color palette colors, with a view to improving the quality of color expressions in color images, there is presented a problem of interchangeability with the existing standards.
Specifically, if the 16/4096 format in which 16 colors are selected from the selectable 4096 colors for color palettes for color expressions is to be extended to, for example, a 256/2.sup.18 format in which 256 colors are selected from 2.sup.18 (about 260,000) colors, it may occur frequently that, if the image data formulated in accordance with the 16/4096 format are to be expressed in colors in accordance with the 256/2.sup.18 format or, conversely, the image data formulated in accordance with the 256/2.sup.18 format are to be expressed in colors in accordance with the 16/4096 format, the colors displayed differ markedly from the colors of the original image.